Late-Quaternary activity of parallel normal faults along the southern margin of the Yuguang Basin in the Shanxi Rift, China and its seismogeological implications

Abstract

The geometric structure of faults, especially normal faults in extensional regions, is often complex. In faulted depression basins, normal faults typically exhibit branching in both bedrock and sedimentary areas. While paleoearthquake history in sedimentary regions can be studied through the trenching method, this method is not applicable to bedrock faults. For bedrock faults, terrestrial Light Detection and Ranging (t-LiDAR) has become an effective tool for analyzing fault scarps. To fully evaluate fault activity, traditional methods should be combined with new techniques. In this study, we focus on the Yu-Guang Basin southern marginal fault (YBSM Fault) in the Shanxi Rift, China. Although previous studies have investigated the fault, the co-seismic slip history and seismogenic potential remain unclear. We analyzed the Nanmazhuang (NMZ) bedrock fault surface using quantitative morphological analysis, aerial, and field surveys, identifying two seismic events with co-seismic dip slips of 3.1 m and 3.4 m. Empirical formulas suggest the bedrock branch fault can produce earthquakes with magnitudes greater than 7.5. At the Xizhuangtou (XZT) site, a Holocene fault was found with a cumulative displacement of 7 m and a throw rate of 0.7–0.85 mm/yr. A paleoearthquake with a co-seismic slip of 0.4–0.5 m occurred after 23.4–22.5 ka in sediments. The complete paleoearthquake sequence indicates an average recurrence interval of ~6.8 ka and an average elapsed time of ~8.5 ka for the sedimentary fault branch. The study integrates data from both fault branches, demonstrating that both are potential seismic risk sources. Considering the bedrock fault's large coseismic slip and capacity for M > 7.5 earthquakes, as well as the longer elapsed time, attention to seismic hazards on the YBSM Fault is critical.